Perhydropolysilazane s (PHPS) are molecules, oligomers or polymers containing only Si, H and N characterized by repeating —SiHx—NH— units (x=0 to 2) and the fact that the silicon atom is only bonded to a N or H atoms. Several methods have been used and described to make PHPS. See, e.g., U.S. Pat. No. 4,395,460 to Gaul; U.S. Pat. No. 4,482,669 to Seyferth et al.; U.S. Pat. No. 5,905,130 to Nakahara et al.; U.S. Pat. No. 6,329,487 to Abel et al.; and Isoda et al., J. of Inorganic and Organometallic Polymers (1992) Vol. 2, Issue 1, pp. 151-160.
U.S. Pat. No. 4,200,666 to Reinberg discloses a method of preparing silicon nitride films by glow discharge from the decomposition of liquid trisilylamine, which is a volatile monomer.
Scantlin et al. disclose pentaborane(9)-catalyzed condensation of silylamines. Chemical Communications, 1971, p. 1246.
Blum et al. disclose a catalytic method for synthesis of oligosilazanes.
Organometallics 1986, 5, 2081-2086. More particularly, HSiMe2NHMe2SiH is reacted with NH3 using Ru3(CO)12 as a catalyst.
US Pat. App. Pub. No. 2013/0017662 to Park et al. discloses a filler for filling a gap including a compound having the formula SiaNbOcHd, wherein 1.96<a<2.68, 1.78<b<3.21, 0≤c<0.19, and 4<d<10. Abstract. The filler is synthesized by reacting a hydrogenated polysilazane or hydrogenated polysiloxane with trisilylamine in pyridine. Id. at paras 0064-0065. The application targets a compound having a N:Si mole ratio between about 0.7 to about 0.95 to reduce film shrinkage. Id. at para 0051.
Typical synthesis of PHPS involves ammonolysis of silanes to form chains containing the H3Si—N(−)—SiH3 units. The ammonolysis method involves the reaction of NH3 with a halosilane, preferably a dihalosilane, as follows:nH2SiX2+2nNH3→(—SiH2—NH—)n+nNH4ClThe linear (—SiH2—NH—)n perhydridopolysilazane formed can be branched by addition of a tri-functional silane like trichlorosilane. See, e.g., US2014/341794 to Hoppe et al. The linear polysilazane can also undergo cross-linking and cyclization by Si—H/N—H elimination to create new Si—N bonds and partially reduce the H content of the perhydridopolysilazane. See, e.g., U.S. Pat. No. 6,329,487 to Abel et al.
US Pat. App. Pub. No. 2014/341794 to Hoppe et al. discloses a process for preparing trisilylamine (TSA) and polysilazanes in the liquid phase, in which ammonia dissolved in an inert solvent is initially introduced in a substoichiometric amount relative to monochlorosilane which is likewise present in an inert solvent.
KR Pat. App. Pub. No. 2014/0087903 to Song et al. discloses a method for manufacturing chlorine free polysilazane polymers by reacting a nucleophilic compound, such as a tertiary amine or ammonia, with an aminosilane having the formula NR3, wherein each R is independently H, SiR′3, alkyl, aryl, alkoxy, aryloxy, alkyl or aryl oxycarbonyl, acyl, or acyloxy groups and each R′ is independently H, alkyl, aryl, alkoxy, or aryloxy groups. Exemplary aminosilanes include (H3Si)NH2, (H3Si)2NH, (H3Si)NMe2, (H3Si)2NMe, (H3Si)3N, (H2MeSi)NH2, (H2MeSi)(H3Si)NH, (H2MeSi)(H3Si)2N, (H2MeSi)2NH, (H2MeSi)3N, (H2PhSi)3N, (H2PhSi)NH2, (H2PhSi)2NH, (H2(MeO)Si)3N, (H2(MeO)Si)2NH, (H(MeO)2Si)2NH, (H(MeO)2Si)3N, (H2(MeO)Si)NH2, (H(MeO)2Si)NH2, (H3Si)2NMe, (H3Si)2NPh, derivatives thereof, or combinations thereof.
U.S. Pat. App. Pub. No. 2015/004421 to Fujiwara et al. discloses an inorganic polysilazane resin having a Si:N ratio of 1.30:1 or more synthesized by (a) ammonolysis of dichlorosilane in an organic solvent to form an oligomer containing both chlorosilane and a NH group, (b) heating the system to polymerize the oligomer obtained in step (a), and (c) terminal treatment of Si—Cl remaining in the resin with ammonia, if necessary.
PHPS made from ammonolysis of (di)halosilanes contain NH and form solids (NH4Cl), which tends to contaminate the PHPS with residual traces of CI. See, e.g., Example 1 of US2015/004421 to Fujiwara et al. In addition, the Si:N ratio is typically below 1.4:1. Id. at para 0028. When these PHPS formulation are used for coatings, the film is usually converted to silicon oxide by exposure at elevated temperature (>400° C., and usually 550-700° C.) to an oxidizing atmosphere (O2, O3, H2O, H2O2). Upon such conversion, the film tends to shrink, which causes issues for usage in silicon oxide gap fill applications. See, e.g., the EMD Performance Materials website demonstrating a 16-18% shrinkage in the PHPS film at curing temperatures ranging from 400° C. to 1000° C. (http://www.emd-performance-materials.com/en/electronic materials/glossary/phps/phps.html). It has been shown that having a higher Si:N ratio in the PHPS reduces the tendency to such shrinkage. See, e.g., US2015/004421 at Tables 1 and 2.
The PHPS made from ammonolysis may also be converted to silicon nitride by heating them to an elevated temperature, typically >700° C., in an inert or nitriding atmosphere. The resulting silicon nitride becomes an oxidation resistant solid, film, fiber, or powdery material. However, thermal curing of PHPS to enable cross linking into a silicon nitride ceramic involves a significant mass loss associated with the elimination of silicon containing groups, such as N(SiH3)3. Gunthner et al., Journal of the European Ceramic Society, 32 (2012) 1883-1892. As a result, film shrinkage is observed when converting N—H containing PHPS into a solid material. The thermogravimetric data of FIG. 2 show that mass loss starts at a temperature below 200° C., and mass loss which is the cause of the material shrinkage. Id.
Thus a need remains for PHPS having a high Si:N ratio for formation of SiO2 coatings exhibiting minimal shrinkage upon oxidation. A need also remains for PHPS that may be converted to solid silicon nitride at a temperature ranging between room temperature (RT) and 600° C., and preferably between RT and 200° C.